1. Field of the Invention
The present invention relates to transmission and reception technology, specifically, the technology of transmitting and receiving data among semiconductor integrated circuits.
2. Description of Related Art
As the semiconductor technology advances, information processing devices each containing a high-density semiconductor integrated circuit have become prevalent. In these information processing devices, data are transmitted through a transmission path among semiconductor integrated circuits. The semiconductor integrated circuit on the transmitting side of data includes a transmitter circuit (hereinafter referred to as “the transmitter circuit”, and the semiconductor integrated circuit on the receiving side of data includes a receiver circuit (hereinafter referred to as “the receiver circuit.”)
Due to a massive volume of data being processed by the information processing devices in recent years, it has become necessary to accelerate the transmission speed of data among the semiconductor integrated circuits in order to achieve a high transmission speed of processing devices. On the other hand, it is also required to reduce the power consumption of information processing devices and to reduce power required for data transmission. Particularly, in the case of mobile terminals such as PDA and mobile phone that have become rapidly prevalent lately, it is rigorously required to reduce the power consumption of component parts, and the component parts responsible for the transmission of display data between the housing and the display panel are not exempt from such requirement.
In the case of data transmission technology by voltage signals, any attempt to accelerate voltage signals results in delays because of parasitic capacity of the transmission path and thus there is a limit to the acceleration of voltage signals. Japanese Patent Application Laid Open Application No., 2001-052598 discloses a technology of accelerating the transmission of data among semiconductor integrated circuits by transmitting data with electric current signals. This technology can contain the effect of parasitic capacitance of the transmission path and achieve a high transmission speed of signals. The technology of providing the power source in the receiving unit without providing one in the transmitting unit is disclosed so that the work of designing the transmitting unit may be facilitated without the necessity of changing the specification on the transmitter side even if the number of receiving units has change.
Specifically, a pair of wiring cables is provided to transmit signals between the transmitter circuit and the receiver circuit, and in the transmitter circuit one of the wiring cables is connected with the ground electrode and the other is kept in the floating state (high-impedance state). This leads to the flow of electric current in the wiring cables extending from the power source provided in the receiver circuit to the ground electrode and no flow of current to the other wiring cables. As a result, complimentary current signals can be transmitted by a pair of wiring cables. This transmission method is referred hereinafter to as “the CMADS (Current Mode Advanced Differential Signaling).
Lately, small display devices such as mobile telephone in particular include the function of decreasing the amount of image data such as subtractive color mode. This function reduces the amount of data transmitted between the housing and the display panel from 18 bits to 3 bits by reducing for example the image data in 260,000 colors to eight colors.
In the case of transferring image data by reducing their amount in this way, a dummy transfer is carried out other than that of the data required for the display of images. While the dummy transfer is going on, no image data are outputted from the transmitting unit.
However, as current continues flowing in the wiring cables between the transmitting unit and the receiving unit in spite of the fact that no image data are transmitted while the dummy transfer is going on, useless power is consumed.
Taking note of this issue, Japanese Patent Laid Open Application No. 2003-323147 displays the technology of reducing power consumption in the transfer of data by the CMADS method. This technology reduces power consumption by cutting off current flowing in the wiring cables when no image data are outputted from the transmitting unit.
FIG. 10 shows a schematic illustration of data transmission by this technology. Between the transmitting unit 210 for transmitting data (V-I conversion circuits 8 and 9 in FIG. 1 of the 2nd patent application) and the receiving unit 220 for receiving data (I-V conversion circuits 21 and 22 in FIG. 1), two transmission paths, i.e., data transmission path 230 and clock transmission path 240 and STP signal line 250 (wiring cables 11 in FIG. 1) are provided.
The data transmission path 230 includes a pair of wiring cables (data line 230a and data line 230b: wiring cables 4a and 4b in FIG. 1) provided between the transmitter circuit 212 in the transmitting unit (V-I conversion circuits 8 and 9 in FIG. 1) and the receiver circuit 222 in the receiving unit 220 (I-V conversion circuit 21 in FIG. 1). The clock transmission path 240 includes a pair of wiring cables (clock line 240a and clock line 240b: wiring cable 5a and 5b in FIG. 1) provided between the transmitter circuit 214 in the transmitting unit 210 (V-I conversion circuit 9 in FIG. 1) and the receiver circuit 224 in the receiving unit 220 (I-V conversion circuit 22 in FIG. 1).
The STP signal line 250 is provided to allow the flow of signals for controlling whether electric current should be allowed to flow in the data transmission path 230 and the clock transmission path 240 (receiver control signal described in Patent Document 2, referred hereinafter to as “STP signal”), and is a CMOS (Complimentary Metal Oxide Semiconductor) signal line. The STP signal is outputted by a timing control circuit not shown. This signal is a signal for indicating whether the transmitting unit is outputting image data.
The receiving unit 220 receives image data by allowing electric current flow in the data transmission path 230 and the clock transmission path 240 when it is receiving this STP signal and the transmitting unit 210 is outputting image data. On the other hand, when the transmitting unit 210 is not outputting image data, it operates in such a way that current may not flow in the data transmission path 230 and the clock transmission path 240. In this way, it is possible to realize a low level of power consumption in the case of image data transmission based on a reduced number of colors.
Also, taking note of the uniform data of each pixel in the case of uniform images such as for example the totally white display, the 2nd patent application discloses a technology of restricting power consumption by cutting off current in the transmission path when the same data continue during a transfer of data. For this operation, the output of image data is interrupted and STP signals are outputted to the receiving unit 220 when the data to be transmitted and the data that had been transmitted a drive timing before are identical. Upon receipt of the STP signal, the receiving unit 220 operates to block the flow of current in the data transmission path 230 and the clock transmission path 240. This results in the subsequent data not being received when the same data continue. In the normal state, the receiving unit 220, generates data for display DATA OUT based on the data received and outputs the same in the image display unit not shown, and when the STP signals are high, the same data as the data generated for display DATA OUT based on the data transmitted a drive timing before are outputted. This process enables to accelerate data transmission, and at the same time enables to contain power consumption by reducing the amount of image data to be transmitted when uniform images such as wholly white display are to be displayed and by interrupting the supply of power to the transmission path when image data are not transmitted.
Incidentally, in the case of mobile phone for example, due to a slender construction of the hinge between the housing and the display panel adopted in consideration of the design aspect, voices are raised requesting to reduce as far as possible the wiring cables for data transmission between the housing and the display panel. In view of such an opinion, the technology described in the 2nd patent application increases the wiring cable for STP signals between the transmitting unit 210 and the receiving unit 220.